To achieve greater productivity, titanium alloy requires cutting
at higher speeds (above 100 m min-1) that affects the tool life and energy
consumption during the machining process. This research work correlates the
wear progression and Specific Cutting Energy (SCE) in turning Ti-6Al-4V
alloy using H13 tools (uncoated carbide) in dry conditions from low to high
cutting speeds. Cutting condition employed in this study were selected from
published wear map developed for titanium (Ti-6Al-4V alloy) with the same
tool. Flank wear growth of the tool has been investigated at different
length of cuts in correlation with the SCE under different cutting
conditions. The useful tool life was found to be shorter at high-speed
machining conditions, thus the end of useful tool life criteria (ISO 3685)
was reached at a much shorter length of cuts as compared to low-speed
machining conditions. The cutting conditions corresponding to high wear rate
also resulted in high SCE. Finally, SCE and wear have been related by a
linear relationship that can be used to monitor wear and/or SCE utilization
during machining. The results help in the selection of appropriate cutting
conditions that will enhance the tool life and minimize SCE consumption
during machining titanium alloy.

1Introduction

Titanium alloys have well-known applications that include automotive,
aerospace as well as biomedical implants owing to their excellent
properties, higher strength to density ratio and remarkable corrosion
resistance (Arrazola et al., 2009; Ezugwu et al., 2003). Unalloyed
titanium that has α phase have lesser strength when compared to
titanium alloys presented Table 1. But when titanium is alloyed with
elements like molybdenum, vanadium and aluminum it shows remarkable
properties like higher strength and hardness that makes them suitable for a
variety of industries as well as other applications. Despite all this,
machining this alloy still is a major challenge to the manufacturers and is
considered as difficult to cut material owing to its properties like
chemical reactiveness and small conductivity values (Jaffery et al.,
2013; Zoya and Krishnamurthy, 2000; Deng et al., 2008). Thus these alloys are
considered as material posing high tool wear, shorter tool life, low
material removal rates, poor surface quality and higher cutting temperatures
(Ezugwu et al., 2003; Hughes et al., 2006; Sun and Guo, 2009).

In recent years, special attention has been paid by several researchers to
overcome major difficulties in cutting titanium alloy due to growing
production demands. These efforts include the improvement of cutting tools
using coating materials (Jaffery and Mativenga, 2012; Li et al.,
2018a; Kuram, 2018; Kumar et al., 2018), identification of the most favorable
process parameters for wear minimization (Li et al., 2018b; Revuru et al.,
2018), optimization of multiple responses (Gupta et al., 2018) and
improving the existing tool design as well as use of cryogenic
machining (Shokrani et al., 2018). One of the prime challenges also
includes a reduction in energy demands during machining titanium alloy.
Material characteristics and the process parameters used in cutting
operation influences the energy consumed during machining (Balogun
and Mativenga, 2014). Therefore, researchers have focused on different
strategies to minimize the energy consumption during machining that includes
the selection of optimum process parameters or the development of effective
numerical models for machine tools (Warsi et al., 2018a, b).

Machining superalloy like titanium and nickel alloy continued as a topic of
interest for industries and researchers all together. Since titanium alloy
sustain their strength at elevated temperatures due to which temperature at
the chip-tool interface can reach up to 1000 ∘C while machining
Ti-6Al-4V at higher cutting speed (Venugopal et al., 2007a, b). As a
result, rapid wear of the cutting tool occurs, and shorter tool life is
observed. Several studies have reported the mechanism of tool wear during
machining titanium alloy. For monitoring the tool wear, wear maps have been
presented by various researchers for selecting cutting condition that
results in best wear performance of the tool. In case of turning titanium
Ti-6Al-4V, a wear map was published (Jaffery and Mativenga, 2009)
that plots the flank wear rate on the cutting speed Vs. feed rate plot. Li
et al. (2012) also reported a detailed experimental study
of tool wear progression during milling of Ti-6Al-4V alloy using carbide
tools at high speed. The study underlined that cutting forces with the
temperature increased as the cutting velocity is increased resulting in
accelerated tool wear growth during machining. In another study, Bermingham
et al. (2011) analyzed cryogenic turning of Ti-6Al-4V
to analyze the effect of feed and depth of cut on tool life, wear, cutting
forces and the chip morphology. Improved tool life was observed as the
cryogenic coolant helped in reducing the heat generated during the machining
process. A detailed study on the tool wear progression for uncoated carbide
tool in dry, wet and cryogenic environment was carried out by Venugopal et
al. (2007b). It was found that cryogenic
cooling improved the tool life significantly at low cutting speed but at
high speed, the coolant could not penetrate properly in the contact zone.
The authors also reported that the smaller contact length was the main
reason for the increase in temperature and hence promoting tool wear in dry
cutting conditions. However, the correlation between the tool wear and SCE
during machining titanium alloy have not been presented by the previous
researchers.

Strategies targeted for energy minimization have also gained a lot of
interests in recent years and several studies have been reported for energy
analysis in various machining processes. Some notable contributions for
energy modeling and cost reduction as well as sustainable manufacturing
strategies for machining can be found in following literature (Mativenga
et al., 2011; Behrendt et al., 2012; Kara and Li, 2011). The Specific
consumption energy (SCE), which is the energy consumed in removing a unit
volume of the material, can also be controlled by machinist through careful
selection of the process parameters as it depends on the machinability of
material and the cutting conditions during operation (Balogun et
al., 2015). As the energy consumption during machining hard materials such
as titanium is also affected by the tool wear, this research focuses on the
effect of the tool flank wear progression on the SCE during dry turning
Ti-6Al-4V in different cutting situations. Wear map published by Jaffery and Mativenga (2009) was used to identify cutting
parameters in three important zones low, moderate and high tool wear zone.
These conditions were used to evaluate the effect of increasing tool wear on
the SCE in turning Ti-6AL-4V titanium alloy.

Titanium alloy machining has not been investigated in term of energy
consumption like other engineering materials. In case of soft materials like
aluminum, it has been reported that the tool wear is negligible and hence
the machining process does not affect the SCE consumed during the actual
cutting process (Warsi et al., 2018b). For the case of
titanium alloy, the tool wear severely affects the energy consumed for the
actual cutting operation. Therefore, current research is carried out to know
the impact of flank wear growth on specific cutting energy in turning
carried out for titanium alloy (Ti-6AL-4V), as the energy consumed is
affected by the tool blunting and wear continuously. As a final point,
knowing the energy demands of difficult to cut materials like titanium
require such study as it is helpful to identify the right combination of
feed and speed that reduces the SCE and promote economic machining. The
energy measured at the beginning of the process and the end of useful tool
life will be different due to wear growth, which will also produce
variations in SCE as well as the surface roughness of the part. The study is
meaningful as it will also provide a strong basis for producing alternative
tool material as well as increasing the machining efficiency. This work
presented here has identified the correlation between wear and SCE in
turning Ti-6AL-4V at different machining conditions.

2Material and Methods2.1Workpiece material and cutting tool

Titanium Ti-6Al-4V bar was used in this study as the workpiece material
having length 300 mm and diameter 70 mm. The mechanical properties and
chemical composition of the workpiece material are given in Table 1
(Hughes et al., 2006; Jaffery et al., 2016) and Table 2. Uncoated CCMW 09
T3 04 H13 cutting inserts (produced by SANDVIK) without chip breaker having
0 rake angle were used to perform turning experiments. Computer numerical
control (CNC) Turning Center ML-300 manufactured by YIDA Precision Machinery
Co., Ltd, having spindle power 15 kW and total power of 26 KW was used to
carry out dry turning. The maximum spindle speed of 3400 rev min-1 could be
achieved using this CNC machine. Experiments were conducted in steps of
equal cutting length in the axial direction, till the end of the useful life
of the tool. Each experimental run used a fresh cutting insert.

Table 2

Composition (wt %) of Ti-6Al-4V alloy used in experiments.

TiVAlFeCuCr

89.444.25.70.150.0030.0023

2.2Measurement of specific cutting energy

The cutting power during machining was measured using clamp-on meter
Yokogawa CW 240. This power analyzer is capable of measuring power, voltage,
current and power factor of the CNC machine and can record it for an
interval of up to 0.1 s. The measurements were done in two-cycle;
recording air cut power (Pair) and actual cut power (Pactual).
This approach has also been used by previous researchers (Warsi et al.,
2018b; Li and Kara, 2011) for measuring and recording power data. Figure 1
shows the Pair and Pactual recorded during a cutting condition.
The difference between the two power gives the power (Pcut) needed for
material removal during the cycle is given in Eq. (1).
1Pcut=Pactual-PairSCE is the energy needed for removing a unit volume of material and
expressed commonly in units of J mm-3. Specific cutting energy was
measured using Eq. (2).
2SCE=Pcut/material removal rate(vfd)

Figure 1

The recorded actual and air cut power to calculate cutting power at
cutting condition (v=50 m min-1, f=0.12 mm per rev and d=1 mm).

Air cut energy was recorded when the machine tool was not involved in
cutting and the machine is ready to perform cutting operation with all its
components electrically energized. Whereas, actual cutting energy recorded
when the tool workpiece was engaged in cutting operation. The methodology
has also been used by previous researchers (Warsi et al., 2018b; Li and Kara, 2011) for estimating SCE.

2.3Wear measurement

Wear of the inserts was analyzed and measured according to ISO standard
3685; standard for tool life testing (ISO, 1993). The rejection
criteria for cutting tools during the experiments remained as localized
flank wear VB=0.3 mm as well as the fracturing and chipping of the
tool cutting edge. Figure 2 shows the wear measurement using an optical
microscope. The wear progression and energy for each cutting condition were
analyzed where machining was interrupted after a cutting length till the end
of useful tool life. The variation in the SCE as the tool wear progresses
was measured and recorded for each experiment as presented in Figs. 1 and
2.

Figure 4

The experimental setup used for turning operation and the SCE
measurement.

The cutting conditions were selected from wear map published previously by
Jaffery and Mativenga (2009), including cutting
condition from the high, moderate and low wear region as highlighted in
Fig. 3. These machining conditions are presented in Table 3. The selected
cutting conditions from the map are well in agreement with the tool
manufacturer recommended conditions (f= 0.08–0.26 mm per rev, V= 0–250 m min-1 and d= 0–4 mm) for H13 cutting inserts (Sandvik, 2015).
The experimental setup used in turning titanium under dry conditions for the
selected conditions is shown in Fig. 4. The clamp on power meter was
mounted at the main control of the CNC machine and power during actual
cutting for each experimental run was recorded at the meter. Similarly, the
tool wear in each experimental run was studied in steps where the machine
was stopped after a specified length of cut and the wear was observed in the
microscope till the end of tool life.

The energy recorded was analyzed for each experimental condition and three
important zones on the cutting power-time graph were observed as shown in
Fig. 5. The first zone corresponds to the start of the machining cycle
where tool-workpiece interaction occurs and thus produces variation in the
power values recorded due to chattering at the start of cutting (Tool-work-piece interaction zone). The second zone corresponds to the stable
energy zone where the tool- workpiece interaction occurs smoothly without
fluctuation in power recorded (stable zone). The third zone represents the
region in which the increased power fluctuation is observed as the tool wear
starts progressing along the flank face till the end of useful tool life. In
order to achieve greater results, machining in this zone must be avoided as
the power demands increase exponentially in the wear progression zone. The
study of energy consumption in the wear progression zone is very necessary
as it affects the power demands required during machining as well as the
integrity of the final produced work part. The results have been presented
for the first time (turning Ti-6Al-4V alloy) to the knowledge of the author,
therefore, machining at such severe condition must be carefully carried out
to achieve sustainable manufacturing while maximizing productivity.

For each machining condition, the tool state and wear were monitored after a
specified length of cut and analyzed using an optical microscope. Similarly,
the SCE was also measured for each step of the experimental condition
performed. The tool flank wear progression observed for eight selected
machining conditions is presented in Fig. 6, indicating the evolution of
the tool flank wear progression (VB) vs. the cutting length. It was observed
that the wear rate increased when the machining conditions were varied from
low to high-speed conditions on the map. The machining was progressed until
the wear at the tool flank side reached an average value of VB≥0.3
for each cutting condition listed in Table 2. The wear rates were high for
all those conditions at which the combination of feed and speed were
selected from high wear zone on the map, as it can be seen for condition 1,
2, 4 and 5. Whereas, condition 6 and 7, selected from lower wear region of
the map, a low wear rate can be observed. SEM images of the tool wear
progression in Table 3, confirms the increase in the wear as the length of
cut is varied. A fresh insert was used for each specified length of cut.
Tool chipping was observed for condition 1 (V=200 m min-1, f=0.12 mm per rev)
when it was used beyond 150 mm length of cut.

4.3Influence of tool wear on SCE

The progressive tool wear was analyzed for all conditions by considering the
variation in progressive SCE in dry turning Ti-6Al-4V alloy. The results for
the tool wear progression for the condition in low wear zone (low speed) was
observed to produce less variation in the SCE. At low speed and feed rates,
the MRR is low which results in less worn out tool and thus produces little
variation in the SCE consumed as shown in Fig. 7a, b. In case of
uncoated carbide tools at low cutting speed, a layer of adhesive material is
formed on the tool face which protects it from wear out (Fan et
al., 2016). This is also confirmed by the SEM image of the tool when used
for low-speed cutting Fig. 8.

Figure 9a, b the conditions were selected from moderate wear zone and thus
cutting operations resulted in moderate progression of the flank wear.
However, the change in the SCE values was noticeable which could be
attributed to the higher wear rate due to adhesion wear. The wear and SCE in
condition 8 show a high values as the feed rate is higher than condition 3.

The change in the wear and SCE resulted from cutting conditions in the high
tool wear zone (high-speed range) are presented in Fig. 10a–d.
Condition 1 and 2 show the highest rate for both wear and energy among the
selected conditions this is because of shear localization and strong
adhesion of the material near edge resulting in localized temperature zone
at high speed (Dudzinski et al., 2002). This increase in
flank wear during cutting with an increase in velocity and feed has overly
been reported by earlier researchers (Venugopal et al., 2007b; Kaplan et
al., 2018). The higher wear rate and hence the variation in SCE in condition
2 (V=125 m min-1, f=0.24) is also because of the higher feed rate that
produces significant temperature at the tool work piece interface
(Bermingham et al., 2011; Li and Liang, 2006). As can be seen from Fig. 10a the SCE increased from 1.36 (value taken when average flank wear 0.3
is reached) to 2.31 (when maximum wear is reached) resulting 69 % increase
in the SCE value because the wear progression trend sharply increases in
thehigh-speed conditions.

When the speed is high the tool is subjected to high thermal stresses which
result in poor cutting performance of the cutting edge. The increase in
temperature with cutting speed is shown in Fig. 11 (adopted from
Fan et al., 2016).

4.4Influence of cutting speed on wear and SCE

The wear progression and SCE with machining time at two cutting condition
are shown in Fig. 12. The tool life observed is significantly high in
Condition 6 (V=50 m min-1, f=0.12 mm per rev) as compared to Condition 1
(V=200 m/min, f=0.12 mm min-1). With increasing the cutting time, for
Condition 1, the average flank wear of the tool accelerated rapidly and
resulted in very high SCE. This shows that at higher cutting speed the tool
life reduces drastically resulting in higher SCE. Thus tool life can be
significantly improved when using the appropriate cutting condition for
machining titanium. The result also shows that variation in the SCE is due
to the cutting conditions selected from different zones, thereby, conditions
from the map must be carefully selected to promote economic production.

Figure 12

The variation in flank wear and SCE with an increase in cutting
speed (Cond. 1 and Cond. 6).

The SCE has a direct correlation with the flank wear, this can be seen from
Fig. 13. There is a significant increase in the values of SCE as the wear
increases beyond the acceptable limit (0.3 mm) and indicates a need to
change the tool. Therefore, this work carries great importance to use SCE as
a method for relating the tool wear and administering the change of tooling
when needed. The tool flank wear can also be modeled in terms of SCE as
shown in the Fig. 13. Using these simplified relationships machinist can
predict the wear and/or SCE, thereby, optimizing the energy consumption
demand during machining operations. The high-speed machining requires
further investigation using the different coated tool as well as also
provides the basis for the development of new tool material. There is also a
case of using different cutting environments (wet, cryogenic and MQL) to
study the tool wear progression and its influence on SCE.

The research work presented here relates the tool wear and the SCE at high,
moderate and low wear condition. Tool wear is a major reason for the
variation in SCE, as the machining process progresses. The wear is
accelerated due to the increase in speed and the feed rate, noticeably
reducing the life of the tool. When the cutting speed is higher, a high SCE
was observed which is related to an abrupt increase in the tool flank wear
because of elevated temperature at the tool and chip edge. However, for
lower cutting speed the SCE was low and thus tools can be used for longer
cuts that lead to higher machining time. To accomplish sustainable machining
goal with efficient resources, processes must be carried out at low SCE
values which will also ensure low wear of the tool at that point.
Additionally, SCE and wear can be related to a machining condition using
simplified relationships. The results stated here can also be used in
selecting machining conditions which help to improve the tool life and in
minimizing the SCE utilization in dry turning of titanium using H13 inserts.
The authors are currently investigating the phenomenon of tool flank wear
and energy using different coolants as the high-speed machining of such
alloy must be investigated for improved results.

Data availability

No data sets were used in this article.

Author contributions

MY and MK designed experiments, AK helped in developing methodology and graphs plotting.
MY and ZK performed experiments supervised by SHIJ on the CNC machine and characterization of tools. LA and RA reviewed drafted paper, suggested amendments and guidance during revisions.

Competing interests

The authors declare that they have no conflict of interest.

Review statement

This paper was edited by Xichun Luo and reviewed by two anonymous referees.